Weight painting doesn't work properly if bones are moved
The good news is your .blend file is fine. I had no trouble weight painting after changing the pose.

If, by any chance you didn't upload your latest version of your project, please make sure Pose Position is enabled, the Armature Modifier is turned on in the Modifier Tab and remove all keyframes or set keyframes for the current pose on the current frame.

I hope this was helpful to you.

On a side note: you should switch the Shading from Look Dev to Solid in Weight Paint Mode, otherwise the textures will make it harder to see the colors from your Weight Paint.